heyjp it was suggested that one should perform guiding calibration on a star that is close to DEC 0. Is that a good idea? Why?
Pull up a planetarium program (like SkySafari). Display an FOV on it. Perhaps a circle or a rectangle that has dimensions of about 2 degrees.
Move that FOV close to 0 degree declination. Note the size of your FOV in degrees of Right Ascension.
Next, move the FOV close to the pole. Notice that the same FOV (your camera sensor) now covers more degrees of Right Ascension. In fact, if your FOV includes the pole itself, it will cover the entire 24 hours in RA!
Now imagine trying to calibrate near the pole. Because of the above, the guide star will barely move on your sensor for each step of the polar axis of the mount direction.
Calibration is a process where you issue a slew command to the telescope mount at the guiding rate, and then turn off the slew after a predetermined time (this is shown in ASIAIR as the "calibration step size" -- that is why "step size" is in milliseconds of time). This then tells the guide algorithm what the "scale" is, for example, how may pixels the guide star has moved for 1000 milliseconds worth of "pulse" given to the RA motor and to the Declination motor. If the scale is large (as in the case of the RA direction near the pole, the same 1000 milliseconds will produce a much larger movement in the declination direction than in the hour angle direction.
It is usually perfectly fine to calibrate near the pole, and then autoguide where you have calibrated. However, if you calibrate near the pole and then move closer to the equator to autoguide, the RA axis will have poor tracking accuracy.
You can minimize the error by making the guide camera angle orthogonal to your mount's axis (i.e., the red and blue lines in ASIAIR guide info (that circle with the inscribed "i") fall right on the ordinate and abscissa). But it is much easier to just calibrate near declination 0, or recalibrate ever time you change to a different target.
Chen